Postage metering systems print and account for postage and other unit value printing such as parcel delivery service charges and tax stamps. These systems have been both electronic and mechanical. Some of the varied types of postage metering systems are shown, for example, in U.S. Pat. No. 3,978,457 for MICROCOMPUTERIZED ELECTRONIC POSTAGE METER SYSTEM, issued Aug. 31, 1976; U.S. Pat. No. 4,301,507 for ELECTRONIC POSTAGE METER HAVING PLURAL COMPUTING SYSTEMS, issued Nov. 17, 1981; and, U.S. Pat. No. 4,579,054 for STAND ALONE ELECTRONIC MAILING MACHINE, issued Apr. 1, 1986. Moreover, other types of metering systems have been developed which involve different printing systems such as those employing thermal printers, ink jet printers, mechanical printers and other types of printing technologies. Examples of these other types of electronic postage meter are described in U.S. Pat. No. 4,168,533 for MICROCOMPUTER MINIATURE POSTAGE METER, issued Sep. 18, 1979; and, U.S. Pat. No. 4,493,252 for POSTAGE PRINTING APPARATUS HAVING A REMOVABLE PRINT HEAD AND A PRINT DRUM, issued Jan. 15, 1985. These printing systems enable the postage meter system to print variable information which may be alphanumeric and graphic type of information.
Card controlled metering systems have also been developed. These systems have employed both magnetic strip type cards and microprocessor based cards. Examples of card controlled metering systems employing magnetic type cards include U.S. Pat. No. 4,222,518 for METERING SYSTEM, issued Sep. 16, 1980; U.S. Pat. No. 4,226,360 for METERING SYSTEM, issued Oct. 7, 1980; and, U.S. Pat. No. 4,629,871 for ELECTRONIC POSTAGE METER SYSTEM SETTABLE BY MEANS OF A REMOTELY GENERATED INPUT DEVICE, issued Dec. 16, 1986. A microprocessor ("smart card") based card metering system providing an automated transaction system employing microprocessor bearing user cards issued to respective users is disclosed in U.S. Pat. No. 4,900,903 for AUTOMATED TRANSACTION SYSTEM WITH INSERTABLE CARDS FOR TRANSFERRING ACCOUNT DATA, issued Feb. 13, 1990. Moreover, systems have also been developed wherein a unit having a non-volatile read/write memory which may consist of a EEPROM is employed. One such system is disclosed in U.S. Pat. No. 4,757,532 for SECURE TRANSPORT OF INFORMATION BETWEEN ELECTRONIC STATIONS, issued Jul. 12, 1988 and U.S. Pat. No. 4,907,271 for SECURE TRANSMISSION OF INFORMATION BETWEEN ELECTRONIC STATIONS, issued Mar. 6, 1990.
Postage metering systems have also been developed which employ encrypted information printed on a mail piece. The postage value for a mail piece may be encrypted together with other data to generate a digital token. A digital token is encrypted information that authenticates the information imprinted on a mail piece including postage values. Examples of postage metering systems which generate and employ digital tokens are described in U.S. Pat. No. 4,757,537 for SYSTEM FOR DETECTING UNACCOUNTED FOR PRINTING IN A VALUE PRINTING SYSTEM, issued Jul. 12, 1988; U.S. Pat. No. 4,831,555 for SECURE POSTAGE APPLYING SYSTEM, issued May 16, 1989; U.S. Pat. No. 4,775,246 for SYSTEM FOR DETECTING UNACCOUNTED FOR PRINTING IN A VALUE PRINTING SYSTEM, issued Oct. 4, 1988; U.S. Pat. No. 4,873,645 for SECURE POSTAGE DISPENSING SYSTEM, issued Oct. 10, 1989; and, U.S. Pat. No. 4,725,718 for POSTAGE AND MAILING INFORMATION APPLYING SYSTEM, issued Feb. 16. 1988.
These systems, which may utilize a device termed a postage evidencing device (PED), employ an encryption algorithm which is employed to encrypt selected information to generate the digital token. The encryption of the information provides security to prevent altering of the printed information in a manner such that any change in the postal revenue block is detectable by appropriate verification procedures.
Typical information which may be encrypted as part of a digital token includes the value of the imprint, the origination zip code, the recipient addressee information (or zip code), the date and a piece count number. These items of information when encrypted with a secret key and imprinted on a mail piece provide a very high level of security which enables the detection of any attempted modification of the postal revenue block, where this information may be imprinted both in encrypted and unencrypted form. These digital token systems can be utilized with both a dedicated printer, that is, a printer that is securely coupled to an accounting module such that printing cannot take place without accounting or in systems employing non-dedicated printers and secure accounting system. In this case, the non-dedicated printer may print the digital token as well as have other utility and be employed to print other information.
Digital tokens need to be computed and printed in the postal revenue block for each mail piece. The digital token transformation (DTT) computation requires a secret key, that has to be protected and updated. One of the more difficult problems with encrypted evidence of postage payment is the key management problem. Indeed, the use two digital tokens (postal and vendor) is described in pending U.S. patent application of Jose Pastor, George M. Brooknet, Robert A. Cordery and Hyung-Kun Kim, Ser. No. 08/133,427 filed Oct. 8, 1993 for MAIL PROCESSING SYSTEM INCLUDING DATA CENTER VERIFICATION FOR MAILPIECES and assigned to Pitney Bowes Inc., the entire disclosure of which is hereby incorporated by reference, now U.S. Pat. No. 5,390,251. In such systems, the digital tokens are usually computed for every mail piece processed. This computation involves taking input data such as piece count, date, origination postal code and postage amount and encrypting this data with secret keys shared by the postage evidencing device (PED) and postal or courier service and by the postage evidencing device and device manufacturer or vendor. This sharing requires coordination of key updates, key protection and other measures commonly referred to as a key management system. The computation of digital tokens takes place upon request to generate tokens by a mailer. This computation is performed by the postage evidencing device. Thus, the postage evidencing device needs to have all the information required for computation, and, most significantly encryption keys. Moreover, refilling the postage evidencing device with additional postage funds also requires separate keys and a management process. In these systems, the process of token generation is accomplished with real time token computation and tokens can be computed for any combination of input parameters allowed by the system.